During transport or storage of substrates, if traces of dust or gaseous impurities present in the surrounding air adhere to the semiconductor wafers or other objects, the subsequent product yield from the affected wafer is lowered. This tendency becomes increasingly noticeable as the degree of integration increases. Accordingly, there is an increased demand for the environment within these transport containers to achieve a high level of cleanliness regarding not only dust but also gaseous impurities.
Currently to produce a clean space for accommodating substrates when transported or stored, microenvironments are purged using an inert gas which is injected into the interior of the container through an inlet port causing the air within the container to exit through an outlet port. These purge gas inlet and outlet ports are generally located on the bottom surface of the container shell and extend from the interior through the shell bottom surface where they interface with the purge gas delivery system. This method uses a check valve and circular PTFE particulate filter located in the purge inlet port to control and filter the incoming purge gas. The inert purge gas then replaces the air and contaminants within the container and the contaminated air is forced out of the container through the purge outlet port or the door opening if the door is removed.
Other methods of delivering purge gas into the container have been designed to provide the improved environment within the container, these prior art attempts have significant drawbacks which precipitate the need for the present invention. For example, U.S. Pat. No. 6,221,163 to Roberson et al. discloses a system and method for molecular contamination control permitting purging a container to desired levels of relative humidity, oxygen, or particulates. The container includes an inlet port and an outlet port, each including a check valve and filter assembly for supplying a clean, dry gaseous working fluid to maintain low levels of moisture, oxygen, or particulates. The inlet port is connected with a gaseous working fluid source and the outlet port is connected to an evacuation system. The integral directional flow check valves operate at very low pressure differentials (such as less than 10 millibar). In one embodiment, flow of purge gas inside the container can be directed towards the substrates with one or more nozzle towers to encourage laminar flow inside the pod. One or more outlet towers, having a similar function to that of the inlet tower may also be provided to encourage laminar flow inside the container. This method has a particulate filter located in the check valve/filter assembly located in the bottom of the tower. The disadvantage of this method is that as the purge gas flows through the orifices of the tower, the distribution of gas decreases from the tower top orifice to the tower bottom orifice. This results in an extended time to evacuate the water and oxygen from the microenvironment.
One goal of purging is to decrease humidity and oxygen levels in the internal volume of the microenvironment. A challenge of purging is to evenly distribute the purge gas fast and effectively over the substrates within the container. Current purging methods, using inlet and outlet ports at the bottom of the container do not meet either requirement of purging fast or effectively because the path of least resistance for the purge gas flow is directly from the inlet to the outlet. Little of the gas flows between the wafers, and the volume between the wafers is a dead zone that is not immediately affected by the purge gas. This results in non-uniform humidity and oxygen levels within the microenvironment. Utilizing the purge tower improves the speed and effectiveness of the purge, but the orifices in the tower still do not evenly distribute the purge gas between the wafers even with the implementation of graduated orifices. In effect, as the purge gas flows through the orifices of the tower, the distribution of gas decreases from the top orifice to the bottom orifice. This results in ineffective evacuation of water and oxygen from the microenvironment.
Another goal of purging is to remove airborne contaminants from the environment within the container. These contaminants are primarily trapped in the filtering mechanism in the purge ports and are effectively removed when the container is purged. However, the containers are reusable and prior to reuse must be cleaned. As a part of the cleaning process, a human operator must remove the purge tower and the purge port/filter mechanisms to keep water from accumulating in these devices. Contaminants are introduced into the microenvironment from the human operator during the process of removing/reinstalling the purge towers and purge ports/filters.
A need therefore exists for an improved purging method which solves the problems of evacuating the wafer container in a fast and efficient manner. In particular, a need exists for a purging method and apparatus that can allow the purge gas to accumulate in the tower so that as the gas accumulates and the pressure inside of the tower increases therefore the gas flows through the tower into the microenvironment and is evenly distributed between the wafers. Another need is to minimize or eliminate human operators from reaching inside the container. Currently, operators must reach inside the container to remove and reinstall the purge towers and purge ports/filters when the container is cleaned and returned to service.